OSB should be grade 3 and 18mm is better than 1mm.
Also if you trim side off the OSB and then screw this to a batten creating the drop around the roof.
You could screw roof to shed, before felting. If more than 1 piece of felt, think wind direction. And lastly put gutter on back to collect rain water.
OSB might be very pricey at the mo.
I used onduline, but not worth the expense for a plot shed.
You could put purlins in to help support new roof, I used 1x2 on mine, especially if OSB is 11mm